During the initial production of petroleum from a subterranean oil formation, the downhole pressure alone may be sufficient to force the well fluid upwardly through the well tubing string to the surface of the well bore. As long as the reservoir pressure is high enough, oil and gas are pushed to a wellbore from which they can be recovered. However, as fluids are removed from the reservoir, the pressure decreases. Once the downhole pressure is dissipated below a minimum level, some form of artificial lift is required to elevate the well fluid in the well bore.
A downhole rod pump is the most common form of artificial lift being used today. Typically, the downhole rod pump is suspended within a tubing string and operably connected to a reciprocating surface unit by a string of sucker rods. The sucker rods extend from the surface downhole to the production zone near the end of production tubing. The sucker rod pump is mounted near the end of the production tubing. The pump is driven by the sucker rod which extends to the surface and is connected to a polished rod. The polished rod reciprocates the rod pump to ultimately cause well fluid to exit at the surface.
Typically, the sucker rod pump is a two-cycle pump. During the upstroke, fluid is lifted upward through the tubing and, during the downstroke, the traveling valve and piston is returned to the bottom of the stroke. Subsurface pumps, such as the sucker rod pumps, are designed to pump incompressible liquid. However, petroleum is frequently a mixture of hydrocarbons that can take the form of natural gas and liquid crude oil. The presence of gas in the pump decreases the volume of oil transported to the surface because the gas takes space that could be occupied by liquid. Thus, the presence of gas decreases the overall efficiency of the pumping unit and reduces oil production. In addition, in wells which produce gas along with oil, there is a tendency for the gas to flow into the pump, which may result in a "gas lock" in the pump whereby no fluid is pumped or elevated in the well bore even though the surface unit is continuing to reciprocate. In the down-stroke of a gas-locked pump, pressure inside a barrel completely filled with gas may never reach the pressure needed to open the traveling valve, and whatever fluid or gas was in the pump barrel never leaves it. However, on the upstroke, the pressure inside the barrel never decreases enough for the standing valve to open and allow the fluid to enter the pump. Thus, for stroke after stroke, no liquid enters or leaves the pump, resulting in a gas-locked condition.
Frequently, a gas locked condition can be avoided by lowering the traveling valve so that a higher compression ratio is obtained in the pump. Lowering the traveling valve to a position close to the standing valve at the bottom of the downstroke will tend to force pump action more often because the traveling valve will open when the traveling valve "hits" the liquid in the pump or when the gas in the pump is compressed to a pressure greater than the pressure above the traveling valve. Lowering the traveling valve near the standing valve does not improve the gas separator efficiency however. If the gas separator does not efficiently separate gas from the liquid that enters the pump, the pump will still perform inefficiently regardless of the traveling valve/standing valve spacing.
In order to prevent entrained gas from interfering with the pumping of the oil, various downhole gas separators have been developed to remove the gas from the well fluid prior to the introduction of the fluid into the pump. For instance, U.S. Pat. No. 3,887,342 to Bunnelle, issued Jun. 3, 1975, and U.S. Pat. No. 4,088,459 to Tuzson, issued May 9, 1978, disclose centrifugal-type liquid-gas separators. U.S. Pat. No. 2,969,742 to Arutunoff, issued Jan. 31, 1961, discloses a reverse flow-type liquid-gas separator. U.S. Pat. No. 4,231,767 to Acker, issued Nov. 4, 1980, discloses a screen-type liquid-gas separator. U.S. Pat. No. 4,481,020 to Lee et al., issued Nov. 6, 1984, discloses a screw type inducer for pressuring and separating a liquid-gas fluid mixture.
Sometimes the pump is located below the producing interval and the natural separation of gas and liquid occurs. Other times, the pump is located in or above the producing interval where gas separation is much more difficult. This gas separator is designed for applications where the pump is located in or above the fluid entry zone.
When a pump inlet is placed above or in the formation gas entry zone, a gas separator with a gas anchor should be used below the pump in order to separate the gas from the liquid in an attempt to fill the pump with liquid instead of gas. With respect to gas anchors, U.S. Pat. No. 4,074,763 discloses a tool to be mounted near the end of the production string that uses a series of concentric conduits for separating gas out of the oil/gas mixture. U.S. Pat. No. 4,366,861 separates an oil/gas mixture by reversing the production fluid flow to liberate free gas.